AN-1496| Application Note

Noise, TDMA Noise, and Suppression Techniques

Noise, TDMA Noise, and Suppression Techniques
Noise and TDMA Noise
The term "noise" is often and loosely used to describe unwanted electrical signals that distort the purity of the desired signal. Some forms of noise are unavoidable (e.g., real fluctuations in the quantity being measured), and they can be overcome only with the techniques of signal averaging and bandwidth narrowing. Other forms of noise (e.g., radio frequency interference and "ground loops") can be reduced or eliminated by a variety of techniques, including filtering and careful attention to wiring configuration and parts location. Finally, there is noise that arises in signal amplification and it can be reduced through the techniques of low-noise amplifier design. Although noise reduction techniques can be effective, it always pays to begin with a system that is free of preventable interference and that possesses the lowest amplifier noise possible1. What follows is a brief summary of different kinds of noise afflicting electronic circuits. Thermal Noise (or Johnson Noise or White Noise) is directly relate
d to temperature manifested by the thermal agitation of electrons in resistors. In the case of loudspeakers and microphones, the source of noise is the thermal motion of the air molecules2. Shot Noise is generated due to random fluctuation in the number of charged carriers when emitted from a surface or diffused from a junction. This noise is always associated with a direct current flow, independent of temperature, and is present in bipolar transistors2. Flicker Noise (or 1/f Noise or Pink Noise) is caused mainly by traps associated with contamination and crystal defects.

National Semiconductor Application Note 1496 Allan Soriano May 2006

These traps capture and release carriers in a random fashion and the time constants associated with the process give rise to a noise signal with energy concentrated at low frequencies3. Burst Noise (Popcorn Noise) is generated by the presence of heavy metal ion contamination and is found in some integrated circuits and discrete transistors. With some bipolar integrated circuits, the popcorn noise was a result of too much doping of the emitter. Reducing the doping level made it possible to eliminate the popcorn noise test completely. This is another type of low-frequency noise3. Avalanche Noise is a form of noise produced by Zener or avalanche breakdown in a pn junction. In avalanche breakdown, holes and electrons in the depletion region of a reverse-biased pn junction acquire sufficient energy to create hole-electron pairs by colliding with silicon atoms3. TDMA Noise ("buzz") in GSM mobile phones is generated from a 217Hz waveform which produces an audible noise when coupled into the audio path and conducts
to the speaker, earpiece, or microphone4. Further details of this type of noise will follow. This application note will specifically address the problem of TDMA Noise customers have encountered while driving mono speakers in their GSM phone designs. Before delving into solutions that minimize this problem, some background describing the operations of a bridge-tied load (BTL) mono amplifier will be reviewed. In the following applicable figures, all resistors are of equal value, R (Figure 1).

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FIGURE 1. Bridge-Tied Load Mono Amplifier

AN-1496

2006 National Semiconductor Corporation

AN201970

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